Reconsidering the Gemini Hotend Chassis
The hotend chassis in use on the current Gemini 3D Printer prototype is fairly successful.
It works and proved to be an interesting thought exercise to get it to a serious level of functionality.
I have been considering some of the drawbacks however and going back to previous concepts that show more promise.
My day job as of late has kept me away from more work on the machine, but I needed to take a bit of a break from it anyway. Sometimes I find that one can keep plugging away at a design, come up with many iterations and then just be sick of looking at and thinking about the whole thing.
The Cons to the Current Hotend Chassis
There was a serious drawback to the design that I discovered when testing the prototype out – it really requires that both hotends are manufactured exactly the same. This can be done if you order both hotends at the same time and specify that is needed.
The other option is to take the shortest one you have and then sand down the PEEK portion of the j head to on the other so that it matches the height.
The crux of the issue is that a standard j head hotend could have up to a 0.2mm variance in the machining of the PEEK portion, which determines the overall height of the hotend. While ordering two that are exactly the same is possible, and modifying one after receiving it is also a possibility, these options are really not ideal.
So that means some system of adjusting the height of the mounting portion on the printer itself would be necessary. Given the size and complexity of the current hotend chassis on the Gemini, adding further adjustment for each hotend starts to become unwieldily large and not aesthetically pleasing.
I also have concerns about accuracy of the current system as well potential wear-and-tear on the soft brass nozzles themselves. Plastic tends to build up a bit on the corner of the glass where the hotend needs to measure the Z height, which is easy enough to remove on a sheet of glass, but it’s an extra factor that can affect accuracy.
I also have concerns about pushing a hot nozzle into a non-glass surface such as PEI plastic.
Hotend removal/swapping on this system complicates the matter further. Currently it involves loosening two bolts and removing the front cover panel which is simple enough in itself. However there are four screw terminals per-hotend that also need to be loosened to disengage the hotend electronics. Care needs to be taken when connecting the hotend again to get the wiring to neatly route to the back of the carriage and avoid touching the print bed.
The Reconsidered Concept
I am investigating a new design which would entail having the individual hotends mounted in a cartridge, much like an inkjet 2D printer. Such a design could have contact points for the electronics and a simple latch or magnetic attachment system.
The Z axis endstop would need to be moved elsewhere and would require a different way of detecting when the bed has moved to it’s zero point.
This was actually the original concept for the Gemini when it was still the K02 early prototype. It loses the ability to automatically find the print bed in relation to the hotend nozzle, but greatly simplifies the design of the X carriage and increases it’s flexibility in terms of usable hotends.
Each hotend cartridge mount point on the x-carriage itself would be vertically adjustable by a small amount via an adjustment screw which would allow for variances in hotend manufacturing.
Changing a hotend cartridge would then involve releasing the cartridge from the x-carriage (with magnets this would be as simple as pulling it off the unit) and pressing the pneumatic connector’s release for the bowden tube. Put a new cartridge in would be the reverse of the process with the additional step of needing to turn one screw to fine-tune the levelling of nozzle with the other one on the printer.
An added benefit to this is if the hotend cooling fan was also mounted in the cartridge, then each cartridge would be a self contained unit. This opens up possibilities of using cartridges containing a different hotend than the standard j head, such as one of the more experimental all-metal hotends should they prove reliable, or whatever new version of j head comes out, should it happen to have a different mounting profile.
On a rather different note, I am also considering changing the colour scheme of the printer from it’s current black to a cleaner looking white – this was also something that was originally intended for this machine but was changed early in the ordering process. If there are early prototype parts of the new concept made, they won’t reflect this per-se, however the new plastic is on order.
Stay tuned, this project is far from dead. 🙂
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